Experimental Studies and Modelling of Sorption and Diffusion of Water and Alcohols in Cellulose Acetate

Abstract

The sorption characteristics of water, methanol and ethanol vapours in cellulose acetate (CA) films were measured by microgravimetry. The sorption isotherms for water vapours in the CA film at different temperatures in the range 20–40°C do not obey the Flory equation over the whole water activity range. At high water activities, the sorption extent increases much faster with the water activity than it should according to the Flory approach. The isotherms over the whole water activity range can be fitted well by the ENSIC model, a new mechanistic model developed to account for the possibility of solvent cluster formation in the polymer material. Similar behaviour was observed for ethanol, which shows a lower tendency to form clusters, but higher affinity to CA. The sigmoidal shape found for the methanol sorption isotherm suggests a strong sorption on CA sites at low methanol activities. The Guggenheim–Anderson–De Boer equa-tion fitted well this isotherm. The diffusion coefficient, which was calculated from Fickian sorption kinetics at different solvent activities by curve fitting, was constant for water but increased with ethanol content in the membrane according to an exponential relationship characterized by a limit diffusion coefficient and a plasticization coefficient. The limit diffusion coefficient for water was two orders of magnitude larger than that for ethanol, but the activation energy for ethanol was twice as large. Methanol diffusion was only Fickian at a low solvent activity; the diffusion coefficient was one order of magnitude lower than that for water. © of SCI.